It is well recognized that the nation's infrastructure, particularly bridges, is subject to continuous deterioration from the elements. Accordingly, the preservation of the nation's infrastructure remains a growing concern. Bridges must be tested for growing and critical flaws because of the danger to the public of catastrophic failure. Ultrasonic flaw detection techniques can be used to detect and locate structural discontinuities, flaws or other differences or weaknesses in such structures. In addition, ultrasonic flaw detection techniques can be used in many other applications, including the interrogation of factory equipment, machinery, airplane wings, and the like.
With ultrasonic flaw detection techniques, a high-frequency mechanical vibrational energy (ultrasonic energy) is introduced into a test object to non-invasively and nondestructively probe the test object. Traditionally, the ultrasonic energy is transmitted to the test object by means of a transducer. Additionally, electromagnetic acoustic transducers (EMATs) could be employed which generate sound inside a conductive material. The transducer comprises an electromechanical device or piezoelectric material which converts the electric energy of a certain frequency into acoustic energy and also convert the acoustic energy back into electric energy of the same frequency.
First, an electrical signal is applied to the transducer which converts the received electric signal into ultrasonic energy. This ultrasonic energy is transmitted by mechanical bond or through a coupling liquid into a test object. The ultrasonic energy then transmits, reflects, and reverberates through the test object. A second ultrasonic transducer converts the ultrasound back into an electrical signal. Typically, the electrical signal is stored, displayed or otherwise processed to analyze the conditions which exist in the test object.
Currently, there are two primary methods employed for ultrasonic flaw detection. The most common method is classified as "pulse-echo." In pulse-echo systems, typically a single transducer is used for both transmitting and receiving the ultrasonic energy. The second method is classified as "through-transmission" or "pitch-catch." In through-transmission systems, separate transmitting and receiving transducers are employed.
In both of the above systems, short pulses of ultrasonic waves are launched as either plane waves or focused waves. To avoid overlapping the signals collected by the receiving transducer, the ultrasound is launched as short pulses of broad bandwidth that are separated far in time so that the reverberations of one pulse die out before the next pulse is transmitted. During the time span between the pulses, the receiving transducer continuously collects the transmitted, reflected, and reverberated ultrasonic energy of the initial short pulse.
The pulse-echo and through-transmission methods described above, while effective in some applications, have certain drawbacks and deficiencies. Importantly, the detectability of flaws is limited by the energy which can be transmitted per repetition time period of the above pulsing sequence. For example, to permit fine time resolution of the received signal, the pulses must be very short as compared to a relatively long repetition time period between pulses. The duration of a typical pulse may be of the order of 0.1 microsecond while a typical repetition time period between pulses may be of the order of 1000 to 10,000 microseconds. Thus, the duty cycle (pulse duration/repetitive time period) is 10.sup.-4 or 10.sup.-5.
Since the energy which can be used to detect flaws is proportional to the square of the voltage applied to the transducer times the duty cycle of the system, the capabilities of the above flaw detection techniques are limited by the dielectric strength (breakdown voltage) of the piezoelectric material. The breakdown voltage is fixed for a given dielectric material. Accordingly, the total energy useable to determine flaws within a structure is severely limited by the low duty cycle. Moreover, due to the limited energy capacity of the above systems, in order to interrogate a large area of a test object, scanning techniques must generally be used to scan the entire structure point-by-point. The scanning technique requires repeated interrogations at multiple positions along the structure to detect all flaws within the structure. Such techniques are typically, time-consuming, labor-intensive and often involve concomitant tear-down. In the case of bridges, the locations where pulse-echo transducers must be placed for adequate interrogation are often inaccessible.
To improve upon the low duty cycle restriction, one prior art method (described in U.S. Pat. No. 4,167,879 issued to Pedersen) transmits a narrow band pseudo-random coded and phase-modulated interrogation signal at a plurality of carrier frequencies toward the object to be tested. The interrogation signal has a plurality of discrete frequency, continuous wave carrier signals which are modulated by a relatively short repeating code signal. In operation, the respective carrier frequencies are discrete, and the carrier frequency is stepped. Different selected cells are sequentially interrogated by looking at different arrival times of the carrier. By cross-correlating the received signal with a delayed replica of the narrow band pseudo-random code, an improvement upon the low duty cycle restriction can be achieved.
The above method, however, also has certain drawbacks and deficiencies. Specifically, the above narrow band method results in interrogation signals which closely resemble sinusoidal waves. When the interrogation signals are demodulated (using frequency division multiplexing), the pseudo-random code is stripped and the information remaining is the difference between the amplitude and phase of the received signal and the original carrier. This difference provides only limited information about the flaws within the system. The above system also suffers from the constraints of requiring a relatively short code word to modulate the carrier signal. If the code word length is too great, the spectral lines in the output spectrum will be too close to each other to be adequately discriminated, and information regarding the flaws within the system will be lost.